Battery enclosures and NEC [RE-wrenches]

[John Raynes]

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Allan,

Thanks for the suggestion about trying to find plywood sheet stock in 
other more suitable dimensions.  I've come up against the "48 in. 
barrier" myself.  I'll have to see what other sizes we can get in this 
region.  There's only so much scrap OSB and plywood a guy can use, 
unless you've got unlimited space in your shop for building out shelves.

I incorporated insulation in battery boxes from the start because 
that's what everyone said you should do.  I wondered myself about the 
value of the insulation, and I've come up with a couple reasons.

You're right about battery temps gradually finding the mean ambient 
temperature.  On the other hand, limiting the amount and rate of the 
temperature swings is not a bad thing.  From my days in manufacturing 
operations, I learned that if you want to stress test any type of 
device to accelerate failures, you subject it to temperature cycling.  
Temperature cycling is a very effective mechanical test, because the 
differences in temperature coefficients of expansion within the device 
create stresses on various connections, and those stresses can work 
connections loose or fracture joints over time.  I don't know how much 
(if any) meaningful benefit is to be had by limiting temperature swings 
on batteries, but it can't hurt.

Another benefit of insulation, within a location that is subject to 
temperature swings, is that the batteries will all change their 
temperatures at virtually the same rate.   Without insulation, the 
batteries on the perimeter will experience faster and greater 
temperature swings than the ones in the middle.  Since we're charge 
compensating all the batteries the same way, better to have them all 
maintain the same temperature so that they stay as well matched as 
possible, in every way.

Also, in many areas of the country, nightime winter temperatures 
regularly dip well below freezing, but average temperatues (except on 
the north sides of buildings) rarely do.  In these cases freeze 
potential is seriously limited by insulation, given the large amount of 
battery thermal mass that is regulating the temps inside the box.

Of course, if the batteries are in an unregulated but well buffered 
space (such as a basement), the ambient swings are going to be very 
gradual, and the benefits I mentioned above probably wouldn't amount to 
much.  If the space is not buffered and the swings can be large, I 
think there's great benefit, even though I can't really quantify it.

John Raynes
RE Solar
Torrey, UT

At 01:22 PM 10/4/2007 -0700, you wrote:

John, maybe a little of this might help.

I use plywood too, because it's familiar, available, and works, and because
I worked 20 years as a carpenter/woodworker in a former life.

For many sets of batteries, 4' x 8' is an awkward dimension. 4 L-16s require
about 50-52" in one dimension, which leaves a lot of scrap. What I have had
great success with is a type of plywood called "Baltic Birch", that is
commonly used for better cabinet drawer carcases. It comes in 5' x 5'
sheets, so the waste is minimal. The 1/2" material has nine plies, so it's
very easy to glue and fasten to and very dimensionally stable. These
features also let it almost double as furniture or cabinetry, as it finishes
well and looks very professional. We get it through lumber wholesalers, not
retail yards; in our area it's delivered. It costs $30-40/sheet, and two
sheets pretty much does one typical box if carefully laid out.

For a relatively inexpensive liner, try "pond liner" at Home Despot. It's
about $15/linear foot, but is some 10' wide, so one length does two boxes. I
have seen both PVC and EPDM sold as pond liner; I think the EPDM is less
toxic to the environment, both in manufacture and disposal.

We put battery boxes indoors whenever we can, in either a heated or at least
a tempered space. This is to maintain effective capacity during winter by
keeping the batteries warmer. We always use a Power Vent to control
ventilation and prevent backdraft. I will often add a toe-kick base to my
battery boxes, similar to a kitchen base cabinet. It makes servicing the
batteries much easier, to be able to stand close to the box.

I have never understood the point of insulating a battery box, and I wish
someone would explain it to me. In RE applications during cold weather, the
charging process creates a negligible amount of heat. If the batteries are
in an unheated space, and there's no source of heat to keep them warm, they
will eventually settle at the average seasonal ambient temperature of the
space where the box is located. Insulation only slows the rate at which the
cool off and warm up; they'll eventually reach the same temperature with or
without the insulation. So what am I missing here?

Allan at Positive Energy


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